Method for scintillation counting and a scintillation counter with adjustable coincidence resolving time

ABSTRACT

A method for scintillation counting and a scintillation counter exploiting a coincidence technique, in which the coincidence resolving time is flexible and can be automatically adjusted to fit the scintillation characteristics of each sample separately. This is accomplished by measuring during a short initial period the pulse length of the scintillation pulses, and adjusting the coincidence resolving time of the coincidence analyzer before the actual counting of the sample.

FIELD OF THE INVENTION

This invention relates to a method and an apparatus for determining theamounts of a radioactive isotope in scintillation samples. Moreparticularly this invention relates to a determination of quench leveland quench correction in a scintillation counter.

BACKGROUND OF THE INVENTION

Scintillation counting of soft beta-emitters like tritium and carbon-14is a very common analytical technique in life sciences. The aim of thistechnique is to accurately determine the activity of one or severalradioactive isotopes dissolved in a special scintillation liquid held ina transparent vial. The scintillation counter can normally count severalhundreds of vials (samples) in an automatic manner without attendance.

The standard scintillation counter comprises a detector compartment forsequentially holding each sample at a time. Normally the detectorcomprises two photomultiplier (pm) tubes simultaneously convertingphoton pulses that are emitted from the sample into electrical pulses.The pm tubes normally work according to the coincidence technique. Inthis technique, the electrical pulses from both tubes are fed into anelectronic circuit, called the coincidence analyzer, which passes pulseson to pulse height analyzers and scalers only if there is a pulse inboth of the two photomultiplier tubas within a certain time period,called the coincidence resolving time. The function of the coincidenceanalyzer can shortly be described as follows: assume that ascintillation pulse causes an analog output pulse at the output of oneof the two pm tubes. At a certain time, the analog output pulse exceedsa certain threshold and sets a logic signal that will prevail for afinite time period, equal to the coincidence resolving time. Normally,the coincidence resolving time is quite short, typically in the order ofabout 15 ns. If during the coincidence resolving time an analog outputpulse from the other pm tube triggers the coincidence analyzer, theanalyzer sets its output gate signal to indicate a coincidence pulse.This output gate signal causes the analog pulse height analyzer toaccept the two analog pulses from the two pm tubes. Normally the twoanalog pulses are summed before further analysis by pulse heightanalyzers and scalers or a multichannel analyzer.

A radioactive disintegration is a fast phenomenon in itself, but theprocess, in which the disintegration energy is transformed into photons,may extend over a considerable time period, e.g. up to a fewmicroseconds. The characteristics of this scintillation pulse, theintensity and its decay rate, depends on the scintillation medium. Inmost media, the decay consists of two parts: the prompt part, which isthe major part, and the slow, or delayed part. The prompt part, whichoriginates from the lower excited singlet states immediately formed atthe disintegration, is so short and instant that most of the photons canbe observed during the first 20 ns after the disintegration. A typicalscintillation pulse is shown in FIG. 1. The slow part, which isdependent on the formation of higher excited and ionized states, mayextend over a considerable time period and photons in this part may notbe noticed by the coincidence analyzer. This fact is of no concern whenthe total number of photons is high, as in that case there be manyphotons in the prompt part and a high probability that both pm tubeswill receive photons within the coincidence resolving time. But if onlya few photons are emitted, the first photon has a high probability tooccur within the prompt part, and the next may occur much later, orwithin the slow part, after the coincidence resolving time. In this casethe coincidence analyzer will not accept this pulse. Thus, if thecoincidence resolving time is short in comparison to the decay rate ofthe slow part, there is a certain chance that a disintegration resultingin only two or three detected photons will not cause a coincidencecondition. This situation arises typically with low energy isotopes liketritium in certain scintillation media. In scintillation counting thismay cause two undesirable effects: 1) the counting efficiency isunnecessarily reduced and 2) the pulse height spectrum is distorted as aresult of a proportion of pulses with low pulse heights being missed. Asimilar problem has been reported by Wonderly and Quint (S. W. Wonderlyand J. F. Quint, PCT WO 89/02089) when measuring solid scintillators ofa certain type. Wonderly and Quint suggested that the coincidenceresolving time should be longer than usual when measuring solidscintillators of the type described in the patent.

In scintillation counting, quenching of the scintillation light is avery important factor to consider. Quenching in the samples means thatthe number of emitted photons is decreased. The counting efficiency,defined as the ratio between the detected pulse rate to thedisintegration rate, is dependent of the degree of quenching. Usuallythe degree of quenching and the counting efficiency has to be determinedfor each sample separately. As the position of the scintillationspectrum on the pulse height scale also is dependent on the degree ofquenching, one can use this measure as a quench index proportional tothe quench level. By the use of an empirical calibration curve (quenchcurve), the counting efficiency is computed from the determined quenchindex. The mean pulse height (MPH) is an often used measure for theposition of the scintillation spectrum as this is relatively easy tocompute. If the coincidence resolving time is short, then the shape ofthe spectrum, and also the mean pulse height will depend on theintensity and the decay rate of the delayed part. The higher theintensity and the slower the decay rate of the delayed part, the morepulses will be lost in the low amplitude region of the spectrum--causingthe counting efficiency to decrease and MPH to increase. FIG. 2 portraysthe general influence of the coincidence resolving time on the shape ofa tritium spectrum. As a result of this effect, a quench curve based onstandards prepared with a solvent having an insignificant delayed partcan not be used for quench correction of samples prepared with a solventhaving a significant delayed part. As an example, FIG. 3 shows fourquench curves prepared with standards based on toluene, xylene,pseudocumene and di-isopropyl-naphthalene. These quench curves weremeasured in a normal scintillation counter with the coincidenceresolving time equal to 15 ns.

Not only the solvent is important in this respect. Also the quenchingagent has an effect on the decay rate of the delayed part. This effectis demonstrated in FIG. 4, which shows the quench curves for standardsbased on toluene, but with two different quenchers: carbon tetrachlorideand acetone.

The two FIGS. 3 and 4 demonstrate a general problem in scintillationcounting: the composition of the quench curve standards has to beexactly the same as for the samples. This is not always possible toaccomplish. In most cases, only one quench curve is generated and usedwith all sorts of samples, causing systematical errors of more or lessunknown magnitude in the computed radioactivity. It is possible todecrease the systematical error by increasing the coincidence resolvingtime. FIG. 5 shows quench curves for the same samples as used in FIG. 3,but now the coincidence resolving time was increased to above 250 ns.FIG. 6 shows quench curves for the same samples as used in FIG. 4, butnow the coincidence resolving time was increased to above 70 ns.

When measuring liquid samples containing low energy isotopes liketritium it is generally necessary that the coincidence resolving time iskept as short as possible in order to reduce the risk for randomcoincidences between single photon events from thermal noise of pm tubesor chemiluminescence in the sample. Therefore, it is not an acceptablesolution to have the coincidence resolving time always excessively long,e.g. 300 ns. A much better solution is to have the instrumentautomatically adjusting the coincidence resolving time according to thepulse lengths of each sample separately. This can be accomplished bymeasuring a value for a parameter that is proportional to the shape orlength of the scintillation pulse and applying an empirical rule toadjust the coincidence resolving time.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical scintillation decay curve and two time intervalsset so that a simple measure for the pulse length can be obtained.

FIG. 2 shows the influence of the coincidence resolving time on theshape of a tritium spectrum.

FIG. 3 shows four quench curves prepared with standards based ontoluene, xylene, pseudocumene and diisopropyl-naphthalene.

FIG. 4 shows the quench curves for standards based on toluene, but withtwo different quenchers: carbon tetrachloride and acetone.

FIG. 5 shows quench curves for the same samples as used in FIG. 3, butwith the coincidence resolving time increased to above 250 ns.

FIG. 6 shows quench curves for the same samples as used in FIG. 4, butwith the coincidence resolving time increased to 72 ns.

FIG. 7 shows a block diagram of a general embodiment of a scintillationcounter according to this invention.

DESCRIPTION OF THE INVENTION

The objective of this invention is a scintillation counter in which thecoincidence resolving time of the coincidence analyzer can be adjustedautomatically for each sample separately in such a way that quenchcorrection can be performed by using one quench curve only. This can beaccomplished by dividing the measurement of the sample into two periods:during the first period the measured pulse length (mpl) of thescintillation pulses produced by the liquid in the sample is determinedand used to adjust the coincidence resolving time according to anempirical relationship; during the second period the sample is measuredaccording to normal procedures with the coincidence resolving time nowfixed.

A general embodiment of this invention is shown in the block diagram inFIG. 7. In this figure, I is a sample to be measured placed in ameasuring compartment, and 2 and 3 are photon detectors comprisingpreamplifiers for detecting the photons emitted by the sample 1. Thedetectors are connected to a coincidence analyzer 4, having anadjustable coincidence resolving time. The outputs of the two detectorsare also connected to a summing amplifier 5, which is connected to apulse shape analyzing means 6, which measures the pulse length of thescintillation pulses and adjusts the coincidence resolving time ofcoincidence analyzer 4. The analyzer 4 and the summing amplifier 5 areconnected to a pulse height analyzer and scaler means 7, which analyzesand counts the pulses that are approved by the coincidence analyzer 4.The device 7 also computes a quench index. The computed quench index andthe measured count rate is transferred to processing means 8, forfurther data reduction and for output to an external device (notindicated in the diagram).

Generally, in all embodiments of this invention, the pulse length can bedetermined either by using the pulses produced by the internalradioisotope dissolved in the sample or by an external gamma-radiatingsource momentarily placed adjacent to the sample in the measuringcompartment during the first measuring period.

EXAMPLES OF EMBODIMENTS

In a first embodiment of this invention, the pulse length is determinedby registering the pulse shape for a number of pulses by using ananalog-to-digital converter to convert each pulse into a digital formthat can be stored as a histogram in a multichannel analyzer, where eachchannel corresponds to a small fraction of time. The weighted mean orfirst moment of the histogram, the slope of the histogram at some partof it, or the channel dividing the histogram into two areas having acertain ratio are examples of possible measures of the pulse length.

In fact, it is not necessary to register the complete pulse histogram inorder to obtain an exact value for the pulse length, but any measurethat is proportional to this value is sufficient. In a secondembodiment, the pulse is integrated in two time intervals, denoted by Aand B in FIG. 1. The intensities of part A (the area under the curvebetween times t₀ and t₁) and part B (the area under the curve betweentimes t₁ and t₂) are measured for a number of pulses, and a mean valuefor the ratio between these two intensities calculated and used as ameasure for the pulse length.

In a third embodiment, during the first measuring period, the ratiobetween the two intensities A and B are computed for each pulseseparately and compared to a predetermined limit. The pulse is directedto either one of two scalers depending on the result of this comparison.The number of pulses collected in the two scalers are then used tocompute a ratio which is proportional to the desired pulse length.

Further embodiments may employ different schemes wherein the mostsuitable coincidence resolving time is found by measuring, during thefirst measuring period, the effect of the coincidence resolving time onthe pulse rate. Thereafter, the final coincidence resolving time couldbe set so that at least 95% of the maximum count rate found during thefirst measuring period is achieved during the second measuring period.

We claim:
 1. Method of scintillation counting comprising the stepsof:measuring sequentially and automatically a plurality of scintillationsamples capable of emitting photons clustered together in scintillationpulses as a result of radioactive decays occurring either inside oroutside of each sample, detecting said scintillation pulses emitted fromeach of said scintillation samples by one or more photodetectors,connecting a coincidence analyzer to said one or more photodetectors,said analyzer having an adjustable coincidence resolving time,connecting one or more incremental scalers to said coincidence analyzerand said one or more photodetectors, incrementing the number in said oneor more scalers when at least one pulse from each of said one or morephotodetectors is present within said coincidence resolving time,determining for each sample separately the pulse length of thescintillation pulses during in first measuring period, and automaticallyadjusting said coincidence resolving time for each sample separately inproportion to the determined pulse length prior to a second measurementof said sample in a second measuring period.
 2. Method according toclaim 1, characterized bydetermining the pulse length by registering thepulse shape for a number of pulses by using an analog-to-digitalconverter to convert each pulse into a digital form for storage ahistogram in a multichannel analyzer, where each channel corresponds toa fraction of time, where the pulse length is the weighted mean or firstmoment of the histogram, the slope of the histogram at some part of its,or the channel dividing the histogram into two areas having a certainratio.
 3. Method according to claim 1, characterized bydetermining thepulse length for each sample separately by measuring for eachscintillation pulse, a first intensity value in a first time intervaland a second intensity value in a second time interval, adding togetherthe number of pulses recorded during said first measuring period, saidfirst intensity value to yield first total intensity and said secondintensity value to yield a second total intensity, computing the pulselength from the values of said first total intensity and said secondtotal intensity.
 4. Method according to claim 1, characterizedbydetermining the pulse length for each sample separately by using afirst and a second incremental scaler, measuring for each scintillationpulse during said first measuring period, a first intensity value in afirst time interval and a second intensity value in a second timeinterval, computing the ratio between said intensities and comparingsaid ratio to a certain predetermined limit, incrementing said firstscaler if said ratio exceeds said limit and incrementing said secondscaler if said ratio does not exceed said limit, and, computing thepulse length from the number of pulses stored in said scalers at the endof said first measuring period, where said first time interval extendsfrom time t₀ to time t₁ and said second time interval extends from thetime t₁ to time t₂ and that t₀ <t₁ <t₂.
 5. Method according to claim 1,characterized bydetermining the pulse length for each sample separatelyby determining a first rate of coincidence events when said adjustablecoincidence resolving time is equal to crt₁, determining a second rateof coincidence events when said adjustable coincidence resolving time isequal to crt₂, computing the pulse length from the ratio between saidfirst rate and said second rate.
 6. Method according to claim 1,characterized bydetermining the pulse length for each sample separately,automatically adjusting said coincidence resolving time for each sampleseparately in proportion to the determined pulse length comprises,determining the rate of true coincidence events produced by radioactivedecays at at least two coincidence resolving times, determining themaximum rate of true coincidence events produced by radioactive decays,automatically adjusting said coincidence resolving time so that a fixedfraction of said true maximum rate is recorded.
 7. Method according toclaim 6, characterized bydetermining for each sample separately the rateof true coincidence events produced by radioactive decays by measuringthe rate of all coincidence events, measuring the rate of randomcoincidence events, equating the true rate of detectable coincidenceevents produced by radioactive decays with the difference between saidrate of all coincidence events and said rate of random coincidenceevents.
 8. Method according to any one of claims 1-7, characterizedbydetermining for each sample separately the pulse length by positioninga gamma-ray emitting source adjacent to said sample for causing Comptonelectrons, and determining the length of the scintillation pulsesproduced by said compton electrons.
 9. Scintillation countercomprising:means for measuring sequentially and automatically aplurality of scintillation samples capable of emitting photons clusteredtogether in scintillation pulses as a result of radioactive decaysoccurring either inside or outside of each sample, one or morephotodetectors detecting said scintillation pulses emitted from each ofsaid scintillation samples, a coincidence analyzer connected to said oneor more photodetectors, said analyzer having an adjustable coincidenceresolving time, one or more incremental scalers connected to saidcoincidence analyzer and said one or more photodetectors, means forincrementing the number in said one or more scalers when at least onepulse from each of said one or more photodetectors is present withinsaid coincidence resolving time, means for determining for each sampleseparately the pulse length of the scintillation pulses during a firstmeasuring period, and means for automatically adjusting said coincidenceresolving time for each sample separately in proportion to saiddetermined pulse length prior to a second measurement of said sample ina second measuring period.
 10. Scintillation counter according to claim9, characterized by thatthe pulse length is determined by registeringthe pulse shape for a number of pulses by using an analog-to-digitalconverter to convert each pulse into a digital form for storage as ahistogram in a multichannel analyzer, where each channel corresponds toa fraction of time, where the pulse length is the weighted mean or firstmoment of the histogram, the slope of the histogram at some part of it,or the channel dividing the histogram into two areas having a certainratio.
 11. Scintillation counter according to claim 9, characterized bythat said means for determining for each sample separately the pulselength comprises,means for measuring for each scintillation pulse, afirst intensity value in a first time interval and a second intensityvalue in a second time interval, means for adding together for a numberof pulses recorded during said first measuring period, said firstintensity value to yield a first total intensity and said secondintensity value to yield a second total intensity, means for computingthe pulse length from the values of said first total intensity and saidsecond total intensity.
 12. Scintillation counter according to claim 9,characterized by that said means for determining for each sampleseparately the pulse length comprises,a first and a second incrementalscaler, means for measuring for each scintillation pulse during saidfirst measuring period, a first intensity value in a first time intervaland a second intensity value in a second time interval, means forcomputing the ratio between said intensities and comparing said ratio toa certain predetermined limit, means for incrementing said first scalerif said ratio exceeds said limit and incrementing said second scaler ifsaid ratio does not exceed said limit, and, means for computing thepulse length from the number of pulses stored in said scalers at the endof said first measuring period, where said first time interval extendsfrom time t₀ to time t₁ and said second time interval extends from thetime t₁ to time t₂ and that t₀ <t₁ <t₂.
 13. Scintillation counteraccording to claim 9, characterized by that said means for determiningfor each sample separately the pulse length comprises,means fordetermining a first rate of coincidence events when said adjustablecoincidence resolving time is equal to crt₁, means for determining asecond rate of coincidence events when said adjustable coincidenceresolving time is equal to crt₂, means for computing the pulse lengthfrom the ratio between said first rate and said second rate. 14.Scintillation counter according to claim 9, characterized by that saidmeans for determining for each sample separately the pulse length andsaid means for automatically adjusting said coincidence resolving timefor each sample separately in proportion to said determined pulse lengthcomprises,means for determining the rate of true coincidence eventsproduced by radioactive decays at at least two coincidence resolvingtimes, means for determining the maximum rate of true coincidence eventsproduced by radioactive decays, means for automatically adjusting saidcoincidence resolving time so that a fixed fraction of said true maximumrate is recorded.
 15. Scintillation counter according to claim 9,characterized by that said means for determining for each sampleseparately the rate of true coincidence events produced by radioactivedecays comprisesmeans for measuring the rate of all coincidence events,means for measuring the rate of random coincidence events, means forequating the true rate of detectable coincidence events produced byradioactive decays with the difference between said rate of allcoincidence events and said rate of random coincidence events. 16.Scintillation counter according to any one of claims 9-15, characterizedby that said means for determining for each sample separately the pulselength comprises a gamma-ray emitting source adjacent to said sample forcausing Compton electrons, and means for determining the length of thescintillation pulses produced by said Compton electrons.